Refactor GitHub webhook processing logic into handlers

[joshsmith]

Problem

There are "classes" of webhooks, some of which pertain to the repository and project and other that don't (installation, e.g.). As CC adds more webhook handlers for various reasons, the should_process? function will become increasingly further away from the handler logic, so this should eventually move down into the event handler logic, with the controller merely being a dispatcher.

#1254 (comment)